The following segment was tested in Scientific Linux 6.2.
Taking the usual precautions as a prerequisite (backing up your system…), check to see if the repo RPM package is listed by executing this Terminal command:rpm -qa |grep -i foo

Which will respond with the name of the matching RPMs–if one of them matches the repo you want to get rid of, after having backed up your system, this Terminal command would remove that repo file:rpm -e foo
where “foo” is the name of the repo you want to get rid of.